209

JOURNAL OF PETROLEUM GEOLOGY

vol. 35, no. 3, July 2012

available online: http:// wileyonlinelibrary.com

www.jpg.co.uk

Flow unit distribution and reservoir modelling in Cretaceous 213 - 236

carbonates of the Sarvak Formation, Abteymour oilfield,

Dezful Embayment, SW Iran

H. Rahimpour-Bonab (University of Tehran), H. Mehrabi,

A. Navidtalab and E. Izadi-Mazidi

The Middle Cambrian succession in the Central Baltic Basin: 237 - 254

geochemistry of oils and sandstone reservoir characteristics

O. Zdanaviciute (Institute of Geology and Geography, Vilnius, Lithuania),

J. Lazauskiene, A. I. Khoubldikov, M. V. Dakhnova and T. P. Zheglova

Biomarker characteristics of crude oils from the 255 - 272

Murzuq Basin, SW Libya

T. A. Hodairi and R. P. Philp (University of Oklahoma, USA)

Pore-lining chlorite cements in lacustrine-deltaic sandstones 273 - 290

from the Upper Triassic Yanchang Formation,

Ordos Basin, China

Xia Zhang (Nanjing University), Chun-Ming Lin, Yuan-Feng Cai,

Chang-Wei Qu and Zhao-You Chen

The tight Danian Ekofisk chalk reservoir formation in the 291-310

South Arne field, North Sea: mineralogy and porosity properties

H. Lindgreen (GEUS, Denmark), A. E. Fallick, F. Jakobsen and N. Springer

International Events 312

Cover:

Photo to right shows a core tray from a well in South Arne field, Danish North Sea.

Cores are composed of Danian chalk of the Ekofisk Formation, a reservoir unit of

secondary but increasing importance. Porosity and permeability in the chalk are

reduced by the presence of both clay minerals and detrital nano-sized quartz, as

illustrated in the SEM image (left, above) and the atomic forced microscopy image (left,

below). See the related paper by Lindgreen et al. on pp. 291-310 of this issue. Photos

by Holger Lindgreen (Geus, Denmark).

210

In this issue...

(continues page 212)

Rahimpour-Bonab et al. (pp. 213-236) investigate the distribution of flow and non-

reservoir units in the mid-Cretaceous Sarvak Formation carbonate succession at Abteymour

oilfield, SW Iran. Here the Sarvak Formation consists of shallow-marine limestones of variable

texture dated as late Albian to mid-Turonian. The study focuses on the uppermost part of

the Sarvak succession within which prominent mid-Turonian and Cenomanian-Turonian

disconformities are recognised corresponding to periods of uplift, exposure and intense

weathering. Using data from three wells, two different methods were used to identify flow

units and to distinguish them from barrier and baffle units. The first approach used flow

zone indicator values; the second was based on stratigraphic modified Lorenz plots. The

results of these methods were found to be broadly comparable. Reservoir and non-reservoir

units at the three well were identified in the upper Sarvak Formation within a sequence

stratigraphic framework (derived from previous studies). A series of new “integrated

reservoir zones” were thus defined and correlated between the well locations. The top-

most Sarvak interval, below the mid-Turonian disconformity, had non-reservoir

characteristics inferred to result from the presence of an overmature karst profile due to

weathering during long-term (4-6 Ma) sub-aerial exposure. Shorter-duration (<1 Ma)

exposure associatd with the Cenomanian-Turonian disconformity, by contrast, resulted in

less intense karstification and the mid-Sarvak succession was found to have correspondingly

higher flow unit qualities.

Limited exploration for hydrocarbons has taken place over the past 50 years or so in

the central part of the Palaeozoic Baltic Basin within an area covering western Lithuania,

NW Poland, the Russian enclave of Kaliningrad together with the adjacent offshore. About

50 small-scale oilfields have been discovered, many now almost depleted, and reservoir

rocks occur in the sandstones of the Middle Cambrian Deimena Group. Zdanaviciute et

al. (pp. 237-254) review the reservoir properties of these sandstones, and report on the

geochemical composition of liquid oils from fields in western Lithuania and Kaliningrad.

The Deimena Group sandstones are variably affected by quartz cement, causing vertical

and lateral variations in reservoir properties at both regional and local scales. Field-scale

variations are probably due to differential cementation around palaeo oil-water contacts

during multiple phases of reservoir charging. Geochemical analyses show that the oils

present in these Middle Cambrian reservoirs have relatively homogenous compositions

(low densities and viscosities, low but variable ashphaltene contents), suggesting a single

source. The analytical data indicate that this source is most probably a marine shale deposited

in suboxic conditions containing marine and paralic organic matter. Two principal oil groups

or genetic families were distinguished; a third group showed intermediate characteristics.

Compositional differences may have been due to differences in precursor source rocks

or thermal maturity of kitchen areas.

Hodairi and Philp (pp. 255-272) report on the compositional and biomarker

characteristics of oil samples from three closely-spaced fields in the Palaeozoic Murzuq

Basin of SW Libya. Principal source rocks here occur in a “hot shale” interval at the base of

211

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212

In this issue... (cont.)

the Tanezzuft Formation dated as earliest Silurian. The oil samples (n = 20) were analysed using

standard organic geochemical techniques including GC and GC-MS of saturate and aromatic

fractions and bulk carbon isotope analyses. The presence of a range of n-alkanes up to C32

showed

that the oils were not significantly biodegraded. Biomarker characteristics and related parameters

indicated that all the sample had a relatively homogenous composition and belong to a single

genetic family; carbon isotopic compositions showed little variations. Source rocks are inferred to

have been clay-rich and to have contained mixed marine/terrigenous OM, and data point to

deposition under mild anoxic conditions although results are not unambiguous. Slight differences

in the maturity of the oils, indicated for example by variations in aromaticity-related maturity

parameters, allowed two sub-families of oils to be distinguished. The sub-families are confined to

different oilfield areas and probably represent source kitchens of differing thermal maturities or

different migration pathways.

Lacustrine-deltaic sandstones in the Upper Triassic Yanchang Formation constitute important

reservoir rocks in the SW Ordos Basin, Central China. Using data from analyses of cores from

five wells in the Zhenjiang area, Zhang et al. (pp. 273-290) investigated diagenetic modification in

these sandstones, which are dominated by fine- to medium-grained feldspathic litharenites and

lithic arkoses. Porosities are reduced as a result of compaction and cementation by kaolinite,

quartz and calcite; later grain dissolution however has improved reservoir characteristics. In addition,

the “early” formation of pore-lining chlorite cement, which inhibited precipitation of quartz cement

on detrital quartz grains, allowed relatively high porosities (up to 16%) to be preserved. Two

generations of chlorite cement were observed, probably resulting from changes in temperature

during diagenesis. Petrographic observations show that pore-lining chlorite was precipitated after

an initial phase of mechanical compaction but before quartz, kaolinite and late calcite cementation.

Dissolution of volcanic clasts, biotites and feldspars is inferred to have provided a source of

material for chlorite precipitation from pore waters.

The principal reservoir unit in the so-called “chalk fields” of the Central North Sea is the

Maastrichtian Tor Formation. However as production from this unit declines, there is increasing

interest in the Danian Ekofisk Formation as a secondary target. The permeability of the Ekofisk

Formation, within which “Tight” and “Porous” intervals have been recognised, is variable but is in

general significantly lower than that of the Tor Formation. On pp. 291-310, Lindgreen et al.

consider the reasons for these variations in chalk reservoir properties at South Arne field in the

Danish North Sea. Data came from conventional analyses of core samples from three wells and

included He-porosity and permeability measurements, scanning electron and atomic force

microscopy, and carbon and oxygen stable isotope measurements. Mineralogical analyses focussed

on the non-calcite (insoluble) residues in the chalk samples. These residues were dominated by

nano-sized quartz particles inferred to be of sedimentary origin, together with clay minerals and

minor dolomite. The residue materials were present in relatively large quantities in the Ekofisk

“Tight” intervals, and nano-quartz was found to block pore throats and to result in significantly

reduced permeabilities. Chalk porosity was reduced where both nano-quartz and clay minerals

were present.